1. Field of the Invention
The present invention relates to an electrode for a fuel cell, and a membrane-electrode assembly and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a fuel such as hydrogen or a hydrocarbon-based material such as methanol, ethanol, natural gas, and the like.
A fuel cell is a clean energy alternative that can replace fossil fuels. In addition, a fuel cell has relatively high power output density and energy conversion efficiency, can operate at room temperature, and can have a relatively small-size that can be tightly sealed. Therefore, a fuel cell can be applicable to a wide array of fields such as non-polluting automobiles, electricity generation systems, and portable power sources for mobile equipment, military equipment, and the like.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte fuel cell has relatively high energy density and high power, but requires extra handling capabilities for processing hydrogen gas (or hydrogen-rich gas) and related accessories, such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce the hydrogen gas as the fuel.
By contrast, a direct oxidation fuel cell has lower energy density than that of the polymer electrolyte fuel cell, but it does not need a fuel reforming processor, and can operate at room temperature due to its relatively low operation temperature.
In a fuel cell system, a stack that generates electricity includes unit cells that are stacked adjacent to one another, and each of the unit cells is formed of a membrane-electrode assembly (MEA) and one or more separators (also referred to as bipolar plates). The membrane-electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”), a cathode (also referred to as an “air electrode” or a “reduction electrode”), and a polymer electrolyte membrane between the anode and the cathode.
Electricity is generated as follows. A fuel is supplied to the anode, adsorbed by catalysts of the anode, and then oxidized to produce protons and electrons. The electrons are transferred into the cathode via an external circuit, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode. Then the oxidant, protons, and electrons react with one another on catalysts of the cathode to produce heat along with water.